1. Chapter 3 Pre-Mining

2. Content Introduction Proposed New Framework for a Conceptual Data Warehouse Selecting Missing Value Point Estimation Jackknife estimate

3. Content Why preprocess the data? Data cleaning Data integration and transformation Data reduction Discretization and concept hierarchy generation Summary

4. Introduction

5. A Conceptual Data Warehouse Architecture

6. Proposed New Framework for a Conceptual Data Warehouse two types of clients - high priority users and normal users

7. Normal Users Accessing Data Source

8. High Priority Users Accessing Data Source

9. Selecting

13. Missing Value

14. Point Estimation refers to a single number is estimated. It is used to guess for an unknown population parameter. Mean is one of point estimation techniques.

15. For example there are 100 students’ records in student file. 99 students’ records have score information. One student’s record has missing value of score. The mean score of these is 70. 70 is selected as a value for this student’s score

16. Jackknife estimate is one of famous point estimation techniques. It is left one data value out from the set of observed values each time and is generated that statistic iteratively

17. Example Given the following set of values {2,4,10,16,21}, determine the jackknife estimate for the mean.

19. Why Data Preprocessing? Data in the real world is dirty –incomplete: lacking attribute values, lacking certain attributes of interest, or containing only aggregate data –noisy: containing errors or outliers –inconsistent: containing discrepancies in codes or names No quality data, no quality mining results! –Quality decisions must be based on quality data –Data warehouse needs consistent integration of quality data

20. Multi-Dimensional Measure of Data Quality A well-accepted multidimensional view: –Accuracy –Completeness –Consistency –Timeliness –Believability –Value added –Interpretability –Accessibility Broad categories: –intrinsic, contextual, representational, and accessibility.

21. Major Tasks in Data Preprocessing Data cleaning –Fill in missing values, smooth noisy data, identify or remove outliers, and resolve inconsistencies Data integration –Integration of multiple databases, data cubes, or files Data transformation –Normalization and aggregation Data reduction –Obtains reduced representation in volume but produces the same or similar analytical results Data discretization –Part of data reduction but with particular importance, especially for numerical data

22. Forms of data preprocessing

23. Data Cleaning Data cleaning tasks –Fill in missing values –Identify outliers and smooth out noisy data –Correct inconsistent data

24. Missing Data Data is not always available –E.g., many tuples have no recorded value for several attributes, such as customer income in sales data Missing data may be due to –equipment malfunction –inconsistent with other recorded data and thus deleted –data not entered due to misunderstanding –certain data may not be considered important at the time of entry –not register history or changes of the data Missing data may need to be inferred.

25. How to Handle Missing Data? Ignore the tuple: usually done when class label is missing (assuming the tasks in classification—not effective when the percentage of missing values per attribute varies considerably. Fill in the missing value manually: tedious + infeasible? Use a global constant to fill in the missing value: e.g., “unknown”, a new class?! Use the attribute mean to fill in the missing value Use the attribute mean for all samples belonging to the same class to fill in the missing value: smarter Use the most probable value to fill in the missing value: inference-based such as Bayesian formula or decision tree

26. Noisy Data Noise: random error or variance in a measured variable Incorrect attribute values may due to –faulty data collection instruments –data entry problems –data transmission problems –technology limitation –inconsistency in naming convention Other data problems which requires data cleaning –duplicate records –incomplete data –inconsistent data

27. How to Handle Noisy Data? Binning method: –first sort data and partition into (equi-depth) bins –then one can smooth by bin means, smooth by bin median, smooth by bin boundaries, etc. Clustering –detect and remove outliers Combined computer and human inspection –detect suspicious values and check by human Regression –smooth by fitting the data into regression functions

28. Simple Discretization Methods: Binning Equal-width (distance) partitioning: –It divides the range into N intervals of equal size: uniform grid –if A and B are the lowest and highest values of the attribute, the width of intervals will be: W = (B-A)/N. –The most straightforward –But outliers may dominate presentation –Skewed data is not handled well. Equal-depth (frequency) partitioning: –It divides the range into N intervals, each containing approximately same number of samples –Good data scaling –Managing categorical attributes can be tricky.

29. Binning Methods for Data Smoothing * Sorted data for price (in dollars): 4, 8, 9, 15, 21, 21, 24, 25, 26, 28, 29, 34 * Partition into (equi-depth) bins: - Bin 1: 4, 8, 9, 15 - Bin 2: 21, 21, 24, 25 - Bin 3: 26, 28, 29, 34 * Smoothing by bin means: - Bin 1: 9, 9, 9, 9 - Bin 2: 23, 23, 23, 23 - Bin 3: 29, 29, 29, 29 * Smoothing by bin boundaries: - Bin 1: 4, 4, 4, 15 - Bin 2: 21, 21, 25, 25 - Bin 3: 26, 26, 26, 34

30. Cluster Analysis

31. Regression x y y = x + 1 X1 Y1 Y1’

32. Data Integration Data integration: –combines data from multiple sources into a coherent store Schema integration –integrate metadata from different sources –Entity identification problem: identify real world entities from multiple data sources, e.g., A.cust-id  B.cust-# Detecting and resolving data value conflicts –for the same real world entity, attribute values from different sources are different –possible reasons: different representations, different scales, e.g., metric vs. British units

33. Handling Redundant Data in Data Integration Redundant data occur often when integration of multiple databases –The same attribute may have different names in different databases –One attribute may be a “derived” attribute in another table, e.g., annual revenue Redundant data may be able to be detected by correlational analysis Careful integration of the data from multiple sources may help reduce/avoid redundancies and inconsistencies and improve mining speed and quality

34. Data Transformation Smoothing: remove noise from data Aggregation: summarization, data cube construction Generalization: concept hierarchy climbing Normalization: scaled to fall within a small, specified range –min-max normalization –z-score normalization –normalization by decimal scaling Attribute/feature construction –New attributes constructed from the given ones

35. Data Transformation: Normalization min-max normalization z-score normalization normalization by decimal scaling Where j is the smallest integer such that Max(| |)<1

37. Data Reduction Strategies Warehouse may store terabytes of data: Complex data analysis/mining may take a very long time to run on the complete data set Data reduction –Obtains a reduced representation of the data set that is much smaller in volume but yet produces the same (or almost the same) analytical results Data reduction strategies –Data cube aggregation –Dimensionality reduction –Numerosity reduction –Discretization and concept hierarchy generation

38. Data Cube Aggregation The lowest level of a data cube –the aggregated data for an individual entity of interest –e.g., a customer in a phone calling data warehouse. Multiple levels of aggregation in data cubes –Further reduce the size of data to deal with Reference appropriate levels –Use the smallest representation which is enough to solve the task Queries regarding aggregated information should be answered using data cube, when possible

39. Dimensionality Reduction Feature selection (i.e., attribute subset selection): –Select a minimum set of features such that the probability distribution of different classes given the values for those features is as close as possible to the original distribution given the values of all features –reduce # of patterns in the patterns, easier to understand Heuristic methods (due to exponential # of choices): –step-wise forward selection –step-wise backward elimination –combining forward selection and backward elimination –decision-tree induction

40. Example of Decision Tree Induction Initial attribute set: {A1, A2, A3, A4, A5, A6} A4 ? A1? A6? Class 1 Class 2 Class 1 Class 2 > Reduced attribute set: {A1, A4, A6}

41. Heuristic Feature Selection Methods There are 2 d possible sub-features of d features Several heuristic feature selection methods: –Best single features under the feature independence assumption: choose by significance tests. –Best step-wise feature selection: The best single-feature is picked first Then next best feature condition to the first,... –Step-wise feature elimination: Repeatedly eliminate the worst feature –Best combined feature selection and elimination: –Optimal branch and bound: Use feature elimination and backtracking

42. Data Compression String compression –There are extensive theories and well-tuned algorithms –Typically lossless –But only limited manipulation is possible without expansion Audio/video compression –Typically lossy compression, with progressive refinement –Sometimes small fragments of signal can be reconstructed without reconstructing the whole Time sequence is not audio –Typically short and vary slowly with time

43. Data Compression Original Data Compressed Data lossless Original Data Approximated lossy

44. Histograms A popular data reduction technique Divide data into buckets and store average (sum) for each bucket Can be constructed optimally in one dimension using dynamic programming Related to quantization problems.

45. Discretization Three types of attributes: –Nominal — values from an unordered set –Ordinal — values from an ordered set –Continuous — real numbers Discretization: *divide the range of a continuous attribute into intervals –Some classification algorithms only accept categorical attributes. –Reduce data size by discretization –Prepare for further analysis

46. Discretization and Concept hierachy Discretization –reduce the number of values for a given continuous attribute by dividing the range of the attribute into intervals. Interval labels can then be used to replace actual data values. Concept hierarchies –reduce the data by collecting and replacing low level concepts (such as numeric values for the attribute age) by higher level concepts (such as young, middle-aged, or senior).

47. Summary Data preparation is a big issue for both warehousing and mining Data preparation includes –Data cleaning and data integration –Data reduction and feature selection –Discretization A lot a methods have been developed but still an active area of research